a. Field of the Invention.
The invention relates to the replication of information on a transmissive optical data storage media having information recorded by means of transparent areas in an opaque layer. In the present invention, the replicated information is copied onto a high resolution silver-halide emulsion which is made into a highly reflective optical data storage medium patterned with areas of low reflectivity.
b. Prior Art.
The fundamental principle behind optical data storage and indeed the fundamental principle behind data storage in general is the presence or absence of some signal. Where data storage is achieved optically, this signal may take the form of light transmission through the medium, reflection or scattering from the medium, or absorption by the medium, or any combination of these. Optical data storage media may contain data which is recorded directly on it with a laser beam, or it may contain data which was recorded on a master with a laser beam and then transferred onto replication media. The replication of optical data storage discs or video discs has been accomplished by several means, including pressing (like phonograph records), normal black and white silver-halide photography, diazo photography and photoresist-type photolithography.
Replicated optical data storage media are used for playback of analog or digital data. This media can take the form of circular discs which rotate under a laser beam, or rectangular plates which are scanned by a laser beam. Currently, the most common optical disc is the video disc used in the video playback system manufactured by Magnavox Corporation, a subsidiary of North American Philips. This disc stores information in the form of pits or depressions on a plastic disc which is coated with aluminum. A laser beam is used to read the rotating disc by detecting changes in phase of the reflected signal as the laser beam passes over the pits. The video disc copies are manufactured by injection molding or by a pressing process similar to that used in the manufacture of phonograph records. The copies have the same plastic bases as records. The mold used for making copies is produced by coating a polished glass disc with a thin metal coating which in turn is coated with photoresist. A laser beam exposes the photoresist with the video signal. After photographic development of the photoresist and etching of the thin metal coating, an electroforming process is usually used to build up the stamping mold. By this procedure, thousands of video disc copies may be produced from a master mold. After a disc copy is formed, it is coated with a thin layer of aluminum in a vacuum system. This disc copy is read by light which is reflected from the mirror-like aluminum surface. It cannot be read by transmitted light.
Another method of making copies from master video discs is to use photographic films or diazo copying films. Such a system has been developed by ARDEV, a subsidiary of Atlantic Richfield Corporation. In this system a master video disc is produced by exposing a rotating disc of unexposed photographic film to a modulated laser beam. The master is then photographically developed black so that light absorptive areas are formed by exposure to the laser. Then by contact printing, the master disc images are transferred to a diazo disc by actinic radiation throug the master disc. Finally, the diazo disc is photographically developed with ammonia fumes, and a copy is created. The diazo disc copy is read by transmitted light as opposed to light reflection as in the Magnavox system mentioned above. In transmissive read, a light source is focussed on the disc as it rotates and a photodetector on the opposite side of the disc detects light variations created by the images on the diazo disc passing in front of the light source. The diazo copy can be described as having been produced by photography. The diazo disc is not reflective and can only be read in transmission.
The same photographic master used to produce the diazo video disc copy could be used to produce a thin chromium optical disc by photolithography, similar to what is done to make chromium photomasks in the semiconductor industry. This process involves coating a substrate with a thin metal layer in a vacuum system, which in turn is coated with photoresist. The photographic master is placed in contact with the photoresist and actinic radiation is used to expose the photoresist through the photographic master, thus transferring the images to the photoresist. The photoresist is then developed so as to create small openings in the photoresist through which the metal coating is etched. After etching of the metal coating, the photoresist is stripped off leaving the etched metal replica of the master video disc. The replica can be read by reflecting light from its mirror-like surface or by transmitting light through it, provided that the substrate is transparent.
At the Symposium on Optical Data Display Processing and Storage which was sponsored by the Society of Photographic Scientists and Engineers, on Jan. 23 through 26, 1979, in Orlando, Fla., a paper was presented by D. G. Howe et al. disclosing a method of replicating a storage medium using contact printing. Specifically, the paper discloses the use of photo-activated dye or diazo-type systems to create light absorptive dots in the diazo which covers a mirror-like aluminized substrate. This disc can be read by reflected light but not by transmitted light.
Rice et al. found that contact prints of micron-sized images could be made without serious loss of resolution is care was taken to reduce reaction (Newton rings) at the emulsion interface (An Experimental Television Recording and Playback System Using Photographic Discs, Rice et al., J. SMPTE, Vol. 79, November 1970, page 997). This was done by employing a thin film of liquid within the approximate index of refraction between the two surfaces. The use of liquids in contact printing has been studied extensively. See: Printing Motion Picture Films Immersed in Liquid, Stoll et al., J. SMPTE, Vol. 66, Oct. 1957, page 607 and Printing Motion Picture Films Immersed in a Liquid: Evaluation of Liquids, Delwiche et al., J. SMPTE, Vol. 67, October 1958, page 678.
In patent application Ser. No. 55,270 Bouldin and Drexler disclose a photographic method for producing data images in a reflective recording and data storage medium. Contact and projection methods of exposure through a photomask are used in the semiconductor industry in the process of producing high resolution black silver emulsion photomasks. Such photomasks are squares five inches on a side and typically contain images of two microns and greater. In comparison, in the optical data storage industry the master and copying photosensitive medium are much larger is size, that is, approaching 12 inches in diameter and the images are smaller--that is, one micron or less. In this case conventional contact printing where the master is in direct contact with the copying photosensitive medium will no longer work owing to uncontrollable small air gaps between the master and the copying material. These gaps lead to inability to produce the smallest geometries and the creation of extraneous Newton ring images. Also, the continual contacting of the master with the copying material will eventually lead to damage of the master. Thus, it would be desirable to expose the copy media through a master without directly contacting it.
An object of the invention was to devise a simple and inexpensive method of creating reflective copies of transmissive master optical data recordings without the use of expensive evaporated or sputtered metal coatings. Another object of the invention was to devise a simple replication method that would create an optical data storage disc that could be read by reflected light or transmitted light without the use of vacuum deposition or metal etching methods. A further object was to devise a method of replicating transmissive optical or video discs onto a reflective high resolution replicating medium which is derived from commercially available high resolution silver-halide emulsions. A further object is to create reflective copies typically up to 12 inches in diameter containing data image one micron or less. A further object is to expose the silver-halide copying medium without directly contacting it with the master.